Sometimes one of our friends email address got hacked and the hacker uses the email account to spam all the contacts, ours happened to be in the list, we need to block the email, this is the way to do it in yahoo mail.
Login the yahoo mail and click the "options" on the upper right corner
Classic look
Mail plus look
Options screen for both version
Click the spam, enter the email address which we want to block and click add then "Save Changes", we will say goodbye to our friends' email address.
Cheers,
Louis